1. Field of the Invention
The present invention concerns a switch having an insulating support on which a first and a second external terminal are arranged, and having a temperature-dependent switching mechanism that, as a function of its temperature, makes between the first and the second external terminal an electrically conductive connection for an electrical current to be conveyed through the switch, and comprises a switching member that changes its geometric shape in temperature-dependent fashion between a closed position and an open position, in its closed position the switching member carrying the current, an actuating member being provided that is connected electrically and mechanically in series with the switching member.
2. Related Prior Art
A switch of this kind is known from U.S. Pat. No. 4,636,766.
The known switch comprises, as the switching member, a U-shaped bimetallic element having two legs of different lengths. Attached to the long leg is a movable contact element that coacts with a switch-mounted countercontact that in turn is connected in electrically conductive fashion to one of the two external terminals.
The shorter leg of the U-shaped bimetallic element is attached to the free end of an actuating member, configured as a lever arm, that at its other end is joined immovably to the housing and is connected in electrically conductive fashion to the other of the two external terminals. The actuating member is a further bimetallic element that is matched with the U-shaped bimetallic element in such a way that when temperature changes occur, the two bimetallic elements deform in opposite directions and thus maintain the contact pressure between the movable contact element and the housing-mounted countercontact.
This switch serves as an interrupter for high currents which result in considerable heating of the bimetallic elements through which they flow, so that ultimately the movable contact element is lifted away from the fixed countercontact. Ambient temperature influences are compensated for by the aforementioned oppositely directed shaping of the bimetallic elements.
The principal disadvantage of this design is that two bimetallic elements, whose temperature characteristics must exactly match with one another, are required; this is difficult and cost-intensive to implement in design terms. In order to compensate for production tolerances, the known switch is moreover mechanically adjusted after assembly, which constitutes a further disadvantage.
Since the two bimetallic elements are of very different geometrical configuration, they also have different long-term stability properties, so that readjustment would in fact be necessary from time to time. This is no longer possible during service, however, the overall result being that long-term stability and therefore operating reliability leave much to be desired.
A further disadvantage with this design is the large overall height necessitated by the U-shaped bimetallic element.
Lastly, a further disadvantage with this switch is that it automatically closes again after cooling off, i.e. has no self-hold function that prevents re-closing and thus reactivation of the electrical device protected by the switch.
Switches with a self-hold function are commonly known; with them, a self-hold resistor is connected between the two external terminals, in parallel with the temperature-dependent switching mechanism. When the switch is in the closed state, the self-hold resistor is electrically short-circuited through the switching mechanism, so that it carries no current. If the switching mechanism opens, however, a residual current flows through the self-hold resistor which thereby heats up, as a function of the applied voltage and its resistance value, to such a point that it holds the temperature-dependent switching mechanism at a temperature above the response temperature, so that it remains open.
The prior art discloses a lot of designs for the self-hold resistor in which a block-shaped PTC resistor is used, resulting in an increase in the geometrical dimensions as compared to a switch exhibiting no self-hold function.
A further disadvantage that is associated with the known switches having a self-hold function consists in the design outlay, which results in cost-intensive switches that are difficult to assemble.
A further disadvantage associated with the switch mentioned at the outset is the fact that the threshold value of the current that results in opening of the switch is determined by the ohmic resistance of the bimetallic element, so that it is difficult to implement different switching current values.
It is already known from the prior art, however, to adjust the current dependency by using a dropping or heating resistor that is connected electrically in series with the temperature-dependent switching mechanism. In the known switches, however, an actuating member in the form of a spring snap disk, etc., through which the electrical current flows, is connected in parallel with the switching member. In other words, in current-dependent switches with a dropping resistor the bimetallic element experiences no current, and the operating current of the electrical device being protected is conveyed through a separate spring element. By selecting the resistance value of this dropping or series resistor, the switching current value can now be adjusted accurately and reproducibly.
It is also the case with the known switches having a series resistor that the design outlay is disadvantageous and assembly of the switches is cost-intensive and time-consuming.
A further current-dependent switch known from EP 0 103 792 B1 has as the switching member a bimetallic spring tongue that is attached to one external terminal and carries at its free end a movable contact element that coacts with a countercontact that is arranged at the free end of an elongated spring element that is attached at the other end to the other external terminal, so that the current flows through the series circuit made up of the spring element and bimetallic spring tongue.
The elastic mounting of the countercontact ensures in this case that there is little mechanical load on the bimetallic spring tongue, since the countercontact deflects in limited fashion when the bimetallic spring tongue changes its geometric shape as a result of a temperature change. This prevents irreversible deformations of the bimetallic spring tongue that might result in a shift in the switching temperature.
One disadvantage of this switch is the fact that during the transition from the closed to the open position, the bimetallic spring tongue, like all bimetallic elements, passes through a "creep" phase in which the bimetallic element deforms in creeping fashion in response to an increase or decrease in temperature, but without yet snapping over from its, for example, convex low-temperature position into its concave high-temperature position. This creep phase occurs whenever the temperature of a bimetallic element approaches the kickover temperature either from above or from below, and results in appreciable conformational changes. In addition, the creep behavior of a bimetallic element can also change, in particular, as a result of aging or long-term operation.
During the opening movement, creep can result in a weakening of the pressure of the contact against the countercontact, thus causing undefined switching states. During the closing movement, the contact can gradually approach the countercontact during the creep phase, which can lead to the risk of arcing.
The problems associated with the creep behavior of a bimetallic element are solved, in a current-dependent switch such as described in the aforementioned U.S. Pat. No. 4,636,766 or in EP 0 103 792, by the fact that the bimetallic spring tongues are equipped with dimples with which the creep phase is not completely but at least for the most part suppressed. These dimples or other mechanical impressions provided for suppressing the creep phase onto the bimetallic element are complex and expensive features which moreover greatly reduce the service life of these bimetallic elements. A further disadvantage of the requisite dimple is that not only different material compositions and thicknesses, but also different dimples, must be used for various power classes and response temperatures.